The proton-translocating sector (F0) of the H ion-pumping ATPase of E. coli is composed of three nonidentical subunits. The specific function of each subunit and the arrangement of these subunits in the membrane is not known. The molecular weight and stoichiometry of subunits in this sector will be determined. The exposure of subunits to either face of the membrane will be detemined by chemical modification with impermeant reagents. We hope to identify the subunits that participate in H ion-conducting activity by examining mutants lacking F0-mediated H ion-translocation for altered subunits. The mechanism of F0-mediated H ion-translocation will be studied with liposomes in which F0 has been reconstituted.